1. Field of the Invention
The present invention relates to a cell discharging circuit for use with a fuel cell when the operation of a fuel cell generation system is interrupted or when a control power supply failure occurs.
2. Description of the Prior Art
As to a fuel cell, it is well known that an electrode is degraded when the temperature of the electrode is high (approximately 170.degree. to 200.degree. C. in operation) and a generation voltage from the fuel cell is 0.8 volt/cell or more (no-load voltage=so-called open voltage=1 to 1.1 volts/cell).
When a fuel cell generation system in an operating state changes into a halt state, the temperature of the fuel cell becomes high and electric power supplied to a load is halted, whereby the fuel cell goes into a no-load state. When this occurs, the generated voltage rises, and a condition for degradation of the fuel cell is established.
When the generation system stops, the fuel supply to the fuel cell is shut off by an electromagnetic valve, or the like. At the same time, fuel gas remains in the body or pipings of the fuel cell.
In order to overcome this problem, it is known to decrease the terminal voltage of the fuel cell to 0.8 volt/cell using a circuit for discharging at the time operation is interrupted, and to consume the remaining gas using a discharging resistor.
FIGS. 1 and 2 show a fixed resistor type discharging circuit and its operational timing chart, respectively.
The circuit for discharging is mainly used for a small capacity (for example, several kW) fuel cell power generation system. The output power of the fuel cell (FC) 1 is converted by a converter (CNV) 2 to an alternating current which is supplied to a load 3. A controlling circuit 4 outputs a halt command to stop operation of the fuel cell 1 using switch 5 (SWD). Power to drive controlling circuit 4 is supplied by a control power source 6.
When the switch 5 operates a normally-opened contact 9a is closed to form the discharging circuit. A discharging resistor 7 (RD) consumes electric power generated by the remaining gas in the fuel cell 1.
FIGS. 3 and 4 show a variable resistor type circuit for discharging and the operational timing chart, respectively. In FIG. 3, the same reference numerals are given to the corresponding elements shown in FIG. 1.
The circuit for discharging shown in FIG. 3 is mainly used for a large capacity (for example, several tens kW or more) fuel cell power generation system. Japanese Patent Application Laid-open No. 61-233974, discloses a main terminal voltage control system of the fuel cell. The terminal voltage of the fuel cell 1 is detected by a voltage detector (VD) 8, and then the detected results are applied to the controlling circuit 4. When the controlling circuit 4 outputs a halt command for the fuel cell 1, the switches (SWD1, SWD2, . . . SWDn) operate and those contacts 9a1, 9a2, . . . 9an are closed. As the switches (SWD1, SWD2 . . . SWDn) are closed respectively and sequentially, the terminal voltage of the fuel cell 1 is gradually decreased.
However, the above-mentioned circuit of FIG. 1 has a disadvantage that a discharging circuit is not formed upon a power failure of the control power source, since the discharging circuit is actuated by the switch 5.
Furthermore, since the discharging circuit of FIG. 3 is a complicated arrangement it is disadvantageous because it is bulky. Furthermore, the discharging circuit is not formed when a power failure occurs in the control power source as in the circuit of FIG. 1.